A 69-year-old woman from Seattle, Washington, died after becoming infected with a rare “brain-eating” amoeba.

The woman, who had developed a chronic sinus infection, was advised by her doctor to use a saline irrigation to clear out her sinuses, according to a case study published in the International Journal of Infectious Diseases. She used tap water filtered though a Brita water purifier.

Nasal irrigation systems, such as the neti pot, use salt and water to flush out the nasal passages and relieve symptoms of congestion. The Centers for Disease Control and Prevention recommends that people use distilled or sterile water, or tap water that has been boiled for one minute and then cooled. These methods are preferred over filtered tap water because they are more effective at reducing the risk of infection from organisms that may be in water, according to the CDC.

While safe to drink, these organisms can be dangerous when introduced into the nasal passages.

After one month of irrigating her sinuses with the filtered tap water, the woman developed a red rash the size of a quarter on one side of her nose, according to the report.

Doctors initially mistook the rash for rosacea, a common skin condition, and prescribed her an ointment, but it did not help her symptoms. The woman visited a dermatologist several times and had biopsies taken, but no definitive diagnosis could be made, the authors wrote.

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One year after developing the rash on her nose, the woman suffered a seizure and was admitted to the hospital. A CT scan showed a 1.5 cm lesion on the woman’s brain, which was thought to be a tumor but had “unusual characteristics,” according to the report. Doctors took a biopsy of the mass and sent the sample to Johns Hopkins University for further testing.

The woman was readmitted to the hospital two weeks later after suffering limb weakness and “altered mental status.” Another CT scan showed the mass was increasing in size, and the neuropathologist at Johns Hopkins suggested it was an amoebic infection. Doctors performed another surgery and found severe necrosis, or the death of cells and tissues — for over a year, a type of amoeba had been eating away at her brain.

Amoebas are tiny, single-celled organisms that can move around but can only be seen with a microscope.

“When I operated on this lady, a section of her brain about the size of a golf ball was bloody mush,” Dr. Charles Cobbs, a neurosurgeon at Swedish Medical Center in Seattle who treated the woman, told the Seattle Times. “There were these amoeba all over the place just eating brain cells.”

Although the woman was given an aggressive course of anti-amoebic drugs, her condition worsened and her family ultimately decided to withdraw life support, according to the report.

Tests performed by the CDC showed that the woman’s brain abscess and nasal lesion tested positive for Balamuthia mandrillaris, an amoeba found naturally in soil and water. Rarely, it can cause a serious infection of the brain and spinal cord called granulomatous amoebic encephalitis. The woman’s case was rare and “extremely unusual,” the report authors wrote.

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Other people have gotten amoeba infections from using non-sterile water for nasal irrigation. And the woman in this case isn’t the first person to die after using the wrong kind of water in a neti pot. In 2011, two people died after using neti pots with tap water contaminated with Naegleria fowleri, another “brain-eating” amoeba.

“Clinicians should be aware that patients who develop a nasal rash after sinus irrigation with non-sterile saline washes might be at high risk of amoeba skin infection,” the authors wrote.

While the woman’s history and diagnosis is consistent with the hypothesis that the amoeba entered her brain this way, the authors noted that they did not test her home’s water supply. So their ability to definitely trace the infection to improper sinus irrigation is “limited.”

However, it’s still important to know how to use saline irrigation devices safely. According to the US Food and Drug Administration, improper use of neti pots can increase your risk of infection. Tap water isn’t adequately filtered or treated for these types of organisms, so it may contain low levels of some types of bacteria or amoebas.

Drinking the water is safe, because your stomach acid kills the germs. “But in your nose, these organisms can stay alive in nasal passages and cause potentially serious infections,” the FDA wrote.

Additionally, you should always follow the instructions on the saline irrigation product box and consult a doctor if you have questions or concerns.